You can't outrun the sun. Not even if you're a 2:46 marathoner like Carwyn Sharp, Ph. D. Last August, the Houston-based exercise scientist was diagnosed with invasive malignant melanoma, the most serious form of skin cancer; it kills about 7,500 Americans every year. Luckily, Sharp's cancerous mole on his right ear and a portion of his lobe were removed before the melanoma had spread. "For me, skin cancer was out of sight, out of mind," says Sharp, 35, who grew up in Australia and wore sunscreen infrequently. "Like a lot of runners, I didn't think about it."

While no studies specifically indicate that runners suffer higher rates of skin cancer, experts believe that outdoor athletes are more prone. "If you run outside with your body exposed to the sun, your skin gets damaged, and that increases your risk of skin cancer," says Peter O'Neill, M. D., a dermatologist and marathoner from Garden City, New York.

Despite such warnings, the same health-conscious people who shun smoky restaurants and fast-food joints don't seem as concerned about the potential danger of running in the sun. A 2005 study reported that 85 percent of outdoor athletes didn't use sunscreen at all.

"I think runners have some resistance to it," Dr. O'Neill says. "Your watch, your replacement drink, your shoes ... these are all things that have a direct impact on the quality of your run. You should be doing things to protect your skin, but that pay-o ff may seem vague and distant."

It was for Sharp—until he was having a chunk of his ear removed. Now that the threat of cancer is real, he's doing all he can to prevent another bout, including running before sunrise and wearing sunscreen even under his clothes. By taking these simple precautions, you too can run safely outdoors this summer.

Wear Sunscreen

Buy a bottle with an SPF of at least 30, and one that protects against the two most damaging forms of ultraviolet light, UVA and UVB rays. (See "Second Skin," page 48, for our picks.) In order for it to bind with the skin, you need to apply it 20 minutes before heading out. If you're going more than two hours, you'll need to reapply it. "A sweat-proof formula may offer some additional protection, but it's not enough to stand up to a two-hour-plus run in the heat," says Brian Adams, M. D., an associate professor of dermatology at the University of Cincinnati. He suggests stashing your sunscreen with your sports drink or carrying a small container of it to reapply. Sharp plans his long runs so he can circle back to his car and spray on more midroute.

Dress Smart

Any shirt is better than running bare-chested or in just a sports bra. But a T-shirt isn't ideal, especially if you burn easily. A white cotton tee offers an SPF of only about 3, and when it gets wet with sweat, it's even less protective, says Dr. Adams. Technical apparel that's labeled as sun protective is ideal. This gear comes with a UPF-rating (ultraviolet protection fabric), which means it shields skin from UVB and UVA rays. If you don't want to retire your favorite duds, wash them in SunGuard (which provides UPF 30 protection that will last 20 washings). Or simply wear sunscreen under your clothes. Even though lighter-colored fabrics feel cooler, darker colors block more ultraviolet rays, Dr. Adams says.

Shield Vulnerable Spots

"The sun's rays are more concentrated on your lips, shoulders, tops of ears, and your head," says Rodney Basler, M. D., associate professor at the University of Nebraska Medical Center. "These areas have a higher incidence of skin cancer." So cover them up: Opt for a shirt with sleeves (the longer the better) and a cap (preferably with neck flaps) instead of a visor. Sharp wears both now.

Eat (Don't Absorb) D

Researchers recently reported that more than 75 percent of Americans could be deficient in Vitamin D, the "sunshine vitamin." This may make some runners feel that their tans are justified. But sunlight isn't the only way to remedy a D defi ciency. Food sources and supplements are effective--and not dangerous. "People who won't wear sunscreen in order to get Vitamin D boggle my mind," says Dr. Adams. "Why would you risk getting skin cancer when you can eat a balanced diet?" If you are going to get exposure, it should be before 10 a. m. or after 4 p. m.

Keep at It

In 2006, a study generated headlines when it reported that marathoners had greater numbers of atypical moles, a possible precursor to melanoma. The authors theorized that a suppressed immune function (the result of long, hard training) could be responsible, leaving some runners to question their habit. However, only 56 percent of the runners in the study reported regular sunscreen use. And a correlation was found between markings and runners. "Not all markings or moles become cancerous," Dr. Basler says. "Besides, running is so positive for us in so many ways. We shouldn't discourage it. We just should be aware of sun exposure as a risk--a preventable risk."

Feel Better

Vitamin D deficiencies can contribute to fractures. Active adults could benefit from 2,000 IU of the vitamin a day--a supplement will ensure you get enough.

Second Skin

Runners have plenty of excuses for not wearing sunscreen—it's too messy, it smells like bug repellent, sock tan lines are sexy (okay, true). But we tested several that go on so smoothly and smell so pleasant, there's no reason to skip it.